doc: ti: k3: Add TIFS Stub documentation

* Add documentation to briefly explain the role of TIFS Stub in relevant
  K3 SoC's.
* Shed light on why TIFS Stub isn't package with the DM firmware itself.
* Modify the platform docs wherever the TIFS Stub documentation applies.
* Also, refactor and add a few new labels to help split the firmware
  documentation chunks. This will make it easier to include them one by
  one wherever applicable

Reviewed-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Dhruva Gole <d-gole@ti.com>
Acked-by: Francesco Dolcini <francesco.dolcini@toradex.com> # verdin-am62